The present invention relates to the signal processing art and, more particularly, to an improved audio amplifier design.
A crucial link in modern high fidelity reproduction systems is the audio amplifier. In a typical installation, the audio amplifier connects between a signal source, such as the preamplified output from a phonograph cartridge, and the loudspeakers. Normally, the amplifier must both voltage and power multiply the input signal in order to drive the loudspeaker to a satisfactory level.
It has been found that listeners can discern various forms of amplifier distortion which, although a small percentage of the reproduced signal, nonetheless constitutes a fatigue inducing irritation. One source of such distortion is clipping induced by input signal levels which drive the amplifier beyond its linear drive region and into saturation of the power output devices. A common approach to combat clipping is to provide very high power amplifiers which are capable of large signal excursions before the output devices driven to saturation. Solid state high power amplifiers are expensive to manufacture due, largely, to the high cost of the high voltage, high power semiconductors which must be employed. Compared to their predecessors, electron tubes, transistors are basically low voltage, high current devices. In addition, they are highly susceptible to permanent damage due to overheating.
High voltage transistors which are capable of handling the power levels required in high power amplifier applications are, thus, difficult to manufacture, resulting in high component cost. In addition, compromises in both gain and bandwidth of high voltage semiconductor devices must be made, in order to achieve the high cost of breakdown voltage rating.
To minimize internal heating of output devices in high power, solid-state amplifiers, the output devices have typically been operated in a class AB mode. A necessary limitation of class AB operation is the production of crossover, or notch distortion as symmetrical push-pull devices make the transition between conducting states. While such notch distortion may be a small percentage of the total signal supplied by the amplifier, it is nonetheless objectionable and fatiguing to listeners.
A further problem with solid state amplifier designs has been the result of stored charge in the base region of the output devices. During turn on of an output device a charge develops which acts to maintain the transistor in its "on" state even though drive current is reduced, or eliminated. The storage effects act to reduce the bandwidth over which transistors can operate, thereby leading to high frequency distortion effects in audio amplifier design and, can significantly increase the internal power produced by a semiconductor, leading to device failure.
To avoid the storage effects suffered by bipolar transistors, many commercial audio amplifier designs employ field effect transistors, such as MOSFETS's for use in the driver and output devices. Such devices are, however, expensive, and often require matched characteristics, further adding to their cost.
A further source of distortion in audio amplifier designs has been due to collector-to-base modulation in the amplifier's voltage gain stage. This modulation, sometimes known as the "Early" effect, can lead to amplifier instability.
Further, in typical installations a connecting cable connects between the output terminals of the amplifier and the input terminals of the loudspeaker. The transmission qualities of the interconnecting cable have been found to affect the fidelity of the overall reproduction system. As such, numerous highly sophisticated and very expensive interconnecting cables are now commercially available.